Hydrocracking unit operations are in widespread use in petroleum refineries to process a variety of feeds. Conventional hydrocracking unit process feeds boil in the range of 370° C. to 520° C. and residue hydrocracking units treat feeds boiling above 520° C. In general, hydrocracking processes split the molecules of the feed into smaller, i.e., lighter molecules having higher average volatility and greater economic value. Additionally, hydrocracking typically improves the quality of the hydrocarbon feedstock by increasing the hydrogen-to-carbon ratio and by removing undesirable organosulfur and organonitrogen compounds. The significant economic benefit derived from hydrocracking operations has resulted in the development of substantial process improvements and improved catalysts with greater activity.
Conventional hydrocracking processes of the prior art subject the entire feedstock to the same hydrocracking reaction zones, necessitating operating conditions that must accommodate feed constituents that require increased severity for conversion, or alternatively sacrifice overall yield to attain desirable process economics.
Mild hydrocracking or single-stage hydrocracking operations, typically the simplest of the known hydrocracking configurations, proceed at operating conditions that are more severe than typical hydrotreating processes, and less severe than typical high pressure hydrocracking. Single or multiple catalysts systems can be used depending upon the nature and quality of feedstock and the product specifications. Multiple catalyst systems can be deployed as a stacked-bed configuration or in a series of reactors. Mild hydrocracking operations are generally more cost effective, but typically result in both a lower yield and reduced quality of the middle distillate products as compared to higher pressure hydrocracking operations.
In a series-flow configuration the entire hydrocracked product stream from the first reaction zone, including light gases, e.g., C1-C4, H2S, NH3, and all remaining hydrocarbons, are sent to a second reaction zone. In two-stage configurations, the feedstock is refined by passing it over a hydrotreating catalyst bed in the first reaction zone. The effluents are passed to a fractionating zone column to separate the light gases, naphtha and diesel products boiling in the temperature range of 36° C. to 370° C. The heavier hydrocarbons boiling above 370° C. are then passed to the second reaction zone for additional cracking.
Conventionally, most hydrocracking processes that are implemented for production of middle distillates and other valuable fractions retain aromatics with boiling points in the range of from about 180° C. to 370° C. Aromatics boiling at temperatures greater than the middle distillate range are also present in the heavier fractions.
In all of the hydrocracking process configurations described above, cracked products, along with partially cracked and unconverted hydrocarbons, are passed to a distillation column for separation into products that include naphtha, jet fuel/kerosene and diesel boiling in the nominal ranges of 36° C.-180° C., 180° C.-240° C. and 240° C.-370° C., respectively, with the unconverted products nominally boiling above 370° C. Typical jet fuel/kerosene fractions, e.g., those having a smoke point >25 mm, and diesel fractions, e.g., having a cetane number >52, are of high quality and well above the worldwide transportation fuel specifications. Although the hydrocracking unit products have relatively low aromaticity, any aromatics that do remain lower the key indicative properties of smoke point and cetane number for these products.
The lower olefins, i.e., ethylene, propylene, butylene and butadiene, and aromatics, i.e., benzene, toluene and xylene, are basic intermediates that are widely used in the petrochemical and chemical industries. Thermal cracking, or steam pyrolysis, is a widely used process for obtaining these compounds in the presence of steam and the absence of oxygen. Feedstocks for steam pyrolysis reactors can include petroleum gases and distillates such as naphtha, kerosene and gas oil. The availability of these feedstocks is usually limited and requires costly and energy-intensive processing for their production in a crude oil refinery.
Studies have been conducted using heavy hydrocarbons as a feedstock to steam pyrolysis reactors. A major drawback in conventional heavy hydrocarbon pyrolysis operations is coke formation. For example, a steam pyrolysis process for heavy liquid hydrocarbons is disclosed in U.S. Pat. No. 4,217,204 in which a mist of molten salt is introduced into a steam pyrolysis reaction zone in an effort to minimize coke formation. In one example using Arabian light crude oil having a Conradson carbon residue (CCR) of 3.1% by weight, the cracking apparatus was able to continue operating for 624 hours in the presence of molten salt. In a comparative example without the addition of molten salt, the steam pyrolysis reactor became clogged and inoperable after just 5 hours because of the formation of coke in the reactor.
In addition, the yields and distributions of olefins and aromatics when heavy hydrocarbons are used as the feedstock to a steam pyrolysis reactor are different than those using light hydrocarbon feedstocks. Heavy hydrocarbons have a higher content of aromatics than light hydrocarbons, as indicated by a higher Bureau of Mines Correlation Index (BMCI) which is a measurement of aromaticity of a feedstock that is calculated as follows:BMCI=87552/VAPB+473.5*(SG)−456.8  (1)
where:                VAPB=Volume Average Boiling Point in degrees Rankine and        SG=specific gravity of the feedstock.        
As the BMCI decreases, ethylene yields are expected to increase. Therefore, highly paraffinic or low aromatic feeds are usually preferred for steam pyrolysis in order to obtain higher yields of desired olefins and to avoid undesirable products and coke formation in the reactor coil section.
Systems and methods for subjecting the hydrocarbon feed to an initial step of aromatic extraction and processing the aromatic-rich and aromatic-lean fractions separately and under different hydrocracking conditions is disclosed in U.S. Pat. Nos. 9,144,752, 9,144,753, 9,145,521 and 9,556,388, the disclosures of which are incorporated herein by reference. The systems and reactions schemes are directed to catalyzed hydroprocessing reactions, in multiple stages and, in some cases, in multiple reaction vessels with a first or second stage.
A problem addressed by the present disclosure is to provide an improved process and system for hydrocracking heavy hydrocarbon feedstocks to produce clean transportation fuels and light olefins that is cost effective and efficient.
A further problem addressed is the optimization of the design and operation of a hydrocracking unit to reduce the severity of the operating conditions and reduce catalyst reactor volume requirements for comparable product quality and outputs.